You think it means

Incorrect use: Jack was nonplussed when his new girlfriend described him as “slovenly” – he had been called that since he was a toddler and took it as a compliment.

It really means

NICOLE FORNABAIO/RD.COM

Confused, surprised

Correct use: But Jack’s brother Will was nonplussed when his new girlfriend described him as slovenly – he had always been neat, even as a toddler. Make sure you know these other words that mean the complete opposite of what you thought.